PBS has been presenting an impressive number of science and nature programs in April. One new series, National Geographic's "Strange Days on Planet Earth," documents some of the odd things going on the natural world in our time.

Shelley says that many of these unusual events are cause for concern. Invasive species in particular are causing natural and economic damage all over the world. He says these are species that aren't native to a particular place, but end up out-competing native species for resources, making the diversity less productive in affected places. Formerly rich with native species, Hawaii and the San Francisco Bay area for instance are now overpopulated with non-native flora and fauna, at the expense of native species.